1. Field of the Invention
The present invention relates to content delivery systems and, more particularly, to a method and apparatus for using long forward error correction (FEC) codes in a content distribution system.
2. Description of the Background Art
The demand for broadband content by business and residential subscribers is continually increasing. Broadband content includes multiple types of communications and data, such as broadcast television channels, video-on-demand, streaming video, multimedia data, Internet access, packet telephony, etc. To meet the increasing demand, it is typically necessary to increase throughput to each subscriber and improve overall quality of service. Current delivery technologies include several variations of digital subscriber line (DSL) technology, which uses telephony facilities, and cable modem systems using cable television facilities and hybrid fiber coaxial (HFC) distribution networks.
Delivery of data services over cable television systems is typically compliant with the Data-over-cable-service-interface-specifications (DOCSIS) standard. The content is typically modulated using quadrature amplitude modulation (QAM). Current cable QAM standards use conventional forward error correction (FEC) techniques to transmit the data downstream. FEC is a system of error control for data transmission where the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. The original information may or may not appear in the encoded output; codes that include the un-modified input in the output are systematic, while those that do not are nonsystematic.
It is desirable to improve the sensitivity of QAM transmission within a cable delivery system by using long FEC codes. For example, a low density parity check (LDPC) code can improve the sensitivity of 256-QAM or 1024-QAM by more than 2 dB. However, the use of such long codes results in considerable latency in the transmission path. Accordingly, there exists a need in the art for a method and apparatus for improved FEC in a content distribution system.